The rationale is we're an amateur trading site with volunteer moderators, not an auction or commercial site. If we don't require prices, then we get threads asking for offers which is basically what EBay is. That's not what we're about. If you want to sell it here, you post a price that you want and you sell. You can take less, but we just don't have the bandwidth to host auctions or the mod time to moderate disputes over auction or "make offer" type sales.